The use of fuses in electrical circuits for the purpose of selecting a specific circuit from an array of components and/or for removing a bad circuit is well known. The fuses change from a conductor to an insulator on "activation", which is defined as any process that causes a change in the state of the fuse. Since the fuse is located at a local spot on the chip, the commonly used techniques for "activating" the fuse are (1) passing a sufficiently large current through the fuse elements; (2) focussing a laser or other energetic beam, and (3) sometimes a combination of both. The fuses are usually made of metallic conductors and occasionally polysilicon. For fuses utilizing metallic conductors, activation refers to the local melting of the fuse element, causing a break in the conductor. The change in electrical resistance in this case is from a low finite value to an extremely high value.
When the fuse is electrically blown, the metallic fuse is subjected to a high voltage pulse lasting in the range of milli-seconds to micro seconds. A high current pulse heats the fuse very rapidly by joule heating (I.sup.2 R), raising its temperature to its melting point and sometimes vaporizing a small part of the fuse. When polysilicon fuses are utilized, the poly silicon thin film is recrystallized to change orientation/grain size etc. leading to an increase in resistance. Metallic fuses are preferred in the industry as they provide a substantial change in resistivity on activation. Metallic fuses can also be laser deleted, by focussing a laser beam onto the fuse and melting the fuse material. When the fuse is integrated in a chip, the electrically blowable fuses are built in the multilevel interconnection (MIC) part of the semiconductor. MIC refers to layers of thin film conductors and insulators used to interconnect silicon devices, to form circuits, and to connect to the signal and power I/O's. The insulators used in MIC are usually thin films of silicon dioxide and silicon nitride deposited by PECVD and sputtering techniques. These insulators are part of a group known as inorganic insulators.
In large scale integration, the signal delay time from the MIC has become considerable. The signal travel delay in the MIC is proportional to the product of the resistance of the conductor carrying the signal (R) and the capacitive load (C) on the conductor from adjacent conductors. This is usually known as the transmission delay and is an important consideration for the designers. It has been recognized that lowering permittivity of the thin film insulators used in MIC can minimize signal delay and therefore is desirable for high speed applications. The permittivity of silicon dioxide is approximately 4.0 depending on the deposition technique. It is quite rare to find inorganic insulators with lower permittivites, whereas several organic insulators have permittivities less than 4.0. However, organic insulators have significantly lower thermal stability than inorganics and integration of fuses that are electrically blowable is difficult. It would be highly desirable to be able to use fuses in conjunction with organic insulators in chip wiring or thin film wiring. Table 1 lists some of the organic insulators along with some inorganic insulators and their thermal and dielectric properties.
TABLE 1 ______________________________________ Thermal Stability/ Material Permittivity Melting Point ______________________________________ SiO2 .about.4.0 .about.1700 Si.sub.3 N.sub.4 .about.7.0 .about.1900.degree. C. Polyimides 2.5 to 4.0 400.degree. C. Teflon 2.0 &lt;300.degree. C. SiCr -- 1650.degree. C. (melting) ______________________________________
It is therefore a feature of the present invention to provide a fuse that can be used with organic insulators in microelectronic parts.
Another feature of the present invention is to provide a process of building a fuse by known microelectronics manufacturing techniques.
Yet another feature of the present invention is to provide a fuse that can be electrically blown.
A further feature of the present invention is to provide an electrical blowing operation that does not degrade the MIC structure and functionality.